A major task in the synthesis of the compounds of formula I is the introduction of the chiral centers in the pyrido[2,1-a]isoquinoline moiety, which in the synthesis according to the PCT Int. Appl. WO 2005/000848 involves late stage enantiomer separation by chiral HPLC. Such a process is however difficult to manage on technical scale. The problem to be solved was therefore to find a suitable process alternative which allows obtaining the desired optical isomer in an earlier stage of the process, affords a higher yield and which can be conducted on technical scale.
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